The present invention relates to an alkali battery, which employs a strongly alkaline liquid as electrolyte and of which aperture of the battery casing is hermetically sealed by a sealing unit chiefly including resin.
The typical construction of a cylindrical alkali battery, for example an alkali dry battery is as shown in FIG. 5, which shows a longitudinal cross-section thereof. Specifically, within a battery casing (positive electrode) 1 of cylindrical shape having a head with a positive electrode terminal 8 projecting at its upper end face and an ornamental label 2 stuck on to its outer circumferential surface, there are inserted pellets of positive electrode mixture 3 which are molded in cylindrical shape and consist of manganese dioxide and graphite added as conductive material. On the inside of this positive electrode mixture 3, there is poured in, separated by a separator 4, a gel-form zinc negative electrode 7 obtained by uniformly dispersing gelling agent and zinc alloy powder in an alkaline electrolyte in which is dissolved potassium hydroxide.
The aperture 1a of battery causing 1 is sealed as follows. In aperture 1a at the bottom of battery casing 1, a rod-shaped negative electrode current collector 10 made of brass is pressed into an insertion hole 9a and a resin sealing element (gasket) 9 on to which is fitted an insulating washer 11 made of metal is fitted thereon. Negative electrode current collector 10 is then covered in electrically contacting fashion by a negative electrode terminal plate 12 contacting its head 10a and a folded-back portion 9b formed on resin sealing element 9 is strongly pressed against negative electrode terminal plate 12 by bending and crimping inwards the edges of the bottom aperture of battery casing 1.
In the resin forming of sealing element 9, as shown in FIG. 6, a cavity 18 constituting a molding space for sealing element 9 is formed by mold assembly of lower metal mold 13, upper metal mold 14 and mandrel metal mold 17, and molten resin 20 passing through a resin passage 19a of annular transverse cross-section of resin injection nozzle 19 is poured into this cavity 18 through a resin injection port 18a formed in annular shape by upper metal mold 14, resin injection nozzle 19 and mandrel metal mold 17. When the resin 20 that has been injected has solidified, the mold assembly constituted by lower metal mold 13, upper metal mold 14 and mandrel metal mold 17 is broken open to obtain a sealing element 9 as described above.
FIG. 7 shows a sealing unit 21 assembled using a resin sealing element 9 formed by the molding steps described above. Sealing unit 21 is assembled by pressing in and inserting negative electrode current collector 10 from the open end on the opposite side to resin injection gate 9c corresponding to resin injection port 18a when molding, into insertion hole 9a in sealing element 9. Insulating washer 11 is then mounted by bringing it into contact with inner seat 9d and outer seat 9e, after which negative electrode terminal plate 12 is placed over insulating washer 11, by bringing its central portion into contact with and mounting it on head 10a of negative electrode current collector 10. In fitting this sealing unit 21 into aperture 1a of battery casing 1, when bending the bottom aperture 1a of battery casing 1 inwards, the folded-back portion 9b of resin sealing element 9 is strongly pushed on to negative electrode terminal plate 12 as shown by the arrow.
Due to their use of a strongly alkaline liquid which is an alkaline aqueous solution of high concentration and large ion conductivity even at low temperature as electrolyte, such alkali batteries are able to withstand severe loading, have large capacity, and excellent low-temperature characteristics, and as a result are employed in equipment where power such as in particular motor drive power is needed. On the other hand, the strongly alkaline liquid that is used as electrolyte, due to its high permeability, is subject to the problem that leakage tends to occur due to creeping. Accordingly, sealing of aperture 1a of battery casing 1 is performed by forcing negative electrode current collector 10 into the insertion hole 9a, setting its external diameter to a value larger than the hole diameter of insertion hole 9a of resin sealing element 9, and bending and strongly crimping the aperture rim of battery casing 1.
However, in the case of the prior art sealing unit 21 shown in FIG. 7, small cracks appear in the resin injection gate 9c when negative electrode current collector 10 is inserted into insertion hole 9a of sealing element 9 by forcing it in from one end aperture at the opposite side to resin injection gate 9c on molding, whilst piercing and breaking flash 9f of resin injection gate 9c that closes the aperture at the other end, thereby widening this by pushing outwards. Since the resin injection gate 9c where these cracks start is arranged in contact with the electrolyte, electrolyte permeates into the cracks.
Also, in the case of high-temperature storage, heat cycle repetition, or prolonged storage at normal temperature, alkali batteries are subject to environmental stress cracking at locations subjected to excessive stress in a high-concentration alkaline aqueous solution (electrolyte) atmosphere. In particular, resin injection gate 9c, due to the fact that resin deterioration tends to occur there because of the presence of residual stress on resin injection when molding, tends to constitute a starting point for the environmental stress cracks referred to above which are generated and develop continuously. For example, where 6,6-nylon is employed as the raw material of the sealing element 9, it is inferred that the high-concentration alkaline aqueous solution is selectively absorbed into non-crystalline portions that are present in the crystalline layer, and cracks are created in the gaps between non-crystalline portions in the spherical crystals due to the joint action of external stress and force of the absorbed alkaline aqueous solution tending to wet and spread.
As a result, due to electrolyte that has permeated into the small cracks generated in the resin injection gate 9c creeping up by the creeping phenomenon between the negative electrode current collector 10 and the hole circumferential surface of insertion hole 9a of sealing element 9, cracks are continuously generated and developed originating from the resin injection gate 9c which acquires residual stress during resin molding. In this way, electrolyte permeates as the cracks develop and eventually leaks to the outside.
The present invention has been devised in view of the above problems, its object being to provide an alkali battery wherein the generation of environmental stress cracks can be reliably prevented by a simple construction and whereby excellent resistance to leakage can be obtained.
According to the present invention, in order to achieve the above object, in an alkali battery wherein, after a negative electrode current collector has been inserted into an insertion hole therein, a resin sealing element and negative electrode terminal plate are successively inserted into an aperture of a battery casing, and the aperture rim of said battery casing is then bent inwards and crimped to seal the aperture of said battery casing, said sealing element is accommodated within said battery casing in an arrangement facing said negative electrode terminal plate, with a resin injection gate corresponding to a resin injection port of a metal mold during resin molding thereof positioned at the aperture end of said battery casing.
With this alkali battery, since the resin injection gate corresponding to the resin injection port of the metal mold during resin molding of the sealing element is of a construction arranged at the aperture end of the battery casing and so not contacting the electrolyte, even if cracks are produced caused by residual stress during molding in the resin injection gate, electrolyte does not penetrate into these cracks, so the cracks do not develop to a sufficient degree to cause leakage of electrolyte. Excellent leakage-resistance performance can thereby be obtained.
Preferably in said invention the negative electrode current collector is forcibly inserted into an insertion hole passing through the central location of the sealing element to extend into the interior of the battery casing and is supported in a cantilevered manner, said insertion hole having a hole diameter smaller than the diameter of negative electrode current collector, and the sealing element has the resin injection gate at the aperture rim at the aperture end of said battery casing in said insertion hole.
In this way, since the metal mold for resin molding of the sealing element is of a construction in which a resin injection port is provided at the hole rim of the insertion hole in the middle of the cavity, resin molding of the sealing element is easy. The negative electrode current collector is forced in from the aperture in the vicinity of the resin injection gate at the insertion hole of the sealing element, and although tiny cracks are produced in the resin injection gate which has residual stress on molding, these cracks are generated in a location on the opposite side to the electrolyte in the sealing element, thus, in contrast to the conventional alkali battery, they do not constitute a starting point for the development of environmental stress cracks due to permeation of electrolyte. Apart from this, leakage due to penetration of electrolyte by creeping between the sealing element and the negative electrode current collector can be reliably prevented since the negative electrode current collector is forced into an insertion hole of the sealing element whose hole diameter is set to be smaller than the diameter of the negative electrode current collector.
Also, according to the invention, the sealing element may be provided with the resin injection gate in its face at the aperture edge side of the battery casing in a side part offset from its center.
As a result, since the resin injection gate is positioned in a side part of the sealing element remote from the insertion hole, there is no possibility of cracks being produced therein when the negative electrode current collector is forced into the insertion hole; consequently the negative electrode current collector can be inserted by smoothly forcing it into the insertion hole.
Furthermore, a construction is desirable in which, in the invention, the aperture on the side adjacent the electrolyte in the insertion hole of the sealing element has a curved hole rim chamfered in radiused shape. Consequently, when the negative electrode current collector is inserted by forcing it into the insertion hole of the sealing element, there is no possibility of excessive stress being applied to the aperture rim of the insertion hole adjacent the electrolyte, so the generation of environmental stress cracks at locations of the sealing element adjacent the electrolyte can be reliably prevented; a further improvement in leakage resistance is thereby achieved.